This question has been asked time and time again. If you do a quick search around the forum you will see that there is no ETA for the stable version for the time being, and it would be wrong to even guess one.
It is definitely an active venture with enough backing to get it through so don't feel like Flarum will be forgotten or neglected. ? The developers post in the Dev Diary: Beta 8 discussion where you can keep track to see what they're up to.
